Which DTMF relay method is advertised when the originating SIP gateway sends an INVITE message with a Call-Info header shown?

Refer to the exhibit.

Which DTMF relay method is advertised when the originating SIP gateway sends an INVITE message with a Call-Info header shown?
A . RFC 2833
B . SIP INFO
C . SIP NOTIFY
D . SIP KPML
E . In-band audio

Answer: C

Explanation:

You can develop user-specific applications that reside on your network entity and have the ability to subscribe for event services supported by the IMG. If the network entity wants the ability to detect an entered DTMF digit (only telephone event of “###” are currently supported) from the TDM-side of a call to the IP side of a call, the entity can subscribe to the IMG for these events and receive SIP NOTIFY events containing the digit event.